Should I uninstall the previous version? (I do NOT)
If so, how? (There is an un-installer however if you installed CC 2014 I would not use uninstaller there is a good chance it may mess with File Association and mess up the one CC 2014 installer changed. I would just delete the CC 14.2.1 File system tree If I wanted the disk space back)
And how do I transfer all my plug-ins like Topaz and OnOne Software? (They should be installed into CC 2014)
Do I have to reinstall ALL of them? (It an additional new install for CC 2014)
Adobe removed the Photoshop Preference for adding an additional Plug-in Path to Photoshop. However I still use that method by adding File System link into Photoshop install trees that link my 32Bit and 64Bit Plug-ins as well as my Photoshop Scripts. All the versions of Photoshop I have install share common additional Plug-in and Script path. There Are some exceptions. I install Adobe optional plug-ins into Photoshop install trees. For Adobe been messing with the interfaces. For example the old plug-ins Web Photo Gallery and Picture Package will still work in CS6 Perpetual but not in Not in CS6 Subscription CC and CC 2014.
Adobe does not do a good regression testing they do not maintain Photoshop so it backwards compatible with previous versions of Photoshop. They remove features they have added and break features. Its a good idea to keep old version and not so old version of Photoshop around.
CC 2014 added and removed features and removes some support.